Flaw detection system using acoustic doppler effect

ABSTRACT

A flaw detection system using acoustic Doppler effect for detecting flaws in a medium wherein there is relative motion between the medium and system includes a transducer, spaced from the medium to be inspected, for introducing to and sensing from the medium an acoustic signal that propagates in the medium at a predetermined frequency; and a detector, responsive to the sensed propagating acoustic signal, for detecting in the sensed acoustic signal the Doppler shifted frequency representative of a flaw in the medium.

This is division of Application Ser. No. 09/028,536, filed Feb. 24,1998, now U.S. Pat. No. 6,715,354.

FIELD OF INVENTION

This invention relates to a flaw detection system using acoustic Dopplereffect for detecting flaws in a medium to be inspected wherein there isrelative motion between the system and medium.

BACKGROUND OF INVENTION

Railroads provide both efficiency and economy in passenger and freighttransportation. Like other transportation modes, however, they are proneto various problems. Statistics show that over the course of thiscentury, the average carload and trainload tonnage has increasedsignificantly. There is also an increasing concentration of traffic onfewer main line tracks. The average length of haul has also risen.Unfortunately, these trends have not been offset with a proportionalincrease in the amount of new rail laid. Consequently, the stress onrails and fatigue related failures may continue to increase. With thenew demands, it is important to assess the rail integrity by detectingrail defects nondestructively and speedily.

Typical defects often found in railroad tracks include transverse andlongitudinal defects in the rail head, web defects, base defects,surface defects as well as other miscellaneous damage such as head wear,corrosion, crushed head, burned rail, bolt hole cracks, head and webseparation.

Nondestructive evaluation of rail tracks may be approached by continuousmonitoring or detailed inspection. In the context of rail assessment,continuous monitoring results in global evaluation of the rail whereasdetailed inspection focuses on a particular area to locate and/orcharacterize a defect in detail.

In continuous monitoring, some techniques for inspection of rail flawsat an intermediate speed are currently available, but the technologylacks efficient monitoring techniques at a high speed comparable to thespeed of a passenger car. One of the limitations on speed is the needfor the transducer to be in contact with the rail. Furthermore, existingdetailed inspection techniques have limited capabilities, primarily dueto poor sensor performance and the requirement of contact with the railsurface.

Currently, surface defects are detected by means of a device called atrack circuit. This device uses the track as part of an electric circuitand uses the resistivity of the rail as an indication of surfacediscontinuities. Another approach is the use of ultrasonic probes incontact with the track surface by a rolling wheel. These techniquesrequire contact with the sensor and the rail. Therefore, they are notquite suitable for high-speed monitoring.

Improved inspection systems are needed in many other applications, forexample, in which there is relative motion between the system and mediumto be inspected such as conveyors, cables, ropes and roadbeds. Presentlyinspection techniques tend to be slow and not so reliable because theytypically use a change in the amplitude of the probe signal to identifya defect or flaw. Amplitude data is not easily repeatable or reliable.

SUMMARY OF INVENTION

It is therefore an object of this invention to provide a flaw detectionsystem using acoustic Doppler effect for detecting flaws in a medium tobe inspected.

It is a further object of this invention to provide such a flawdetection system using acoustic Doppler effect which is faster and morereliable.

It is a further object of this invention to provide such a flawdetection system using acoustic Doppler effect which is adapted fordetecting flaws in a variety of moving and stationery mediums such asconveyors, cables, ropes, railroad tracks and roads.

It is a further object of this invention to provide such a flawdetection system using acoustic Doppler effect which utilizes a changein frequency not amplitude to identify a flaw.

It is a further object of this invention to provide such a flawdetection system using acoustic Doppler effect which is capable ofextremely high speed operation and improves its resolution with speed.

It is a further object of this invention to provide such a flawdetection system using acoustic Doppler effect which operates in aremote or non-contact mode spaced from the medium to be inspected.

It is a further object of this invention to provide such a flawdetection system using acoustic Doppler effect which can be used todetect surface or internal flaws.

It is a further object of this invention to provide such a flawdetection system using acoustic Doppler effect in which stronger signalscan be obtained in surface flaw inspection due to air coupling ofacoustic signals.

It is a further object of this invention to provide such a flawdetection system using acoustic Doppler effect which enables continuousnon-stop monitoring.

The invention results from the realization that a truly elegant yetextremely reliable continuous and high speed detection system fordetecting a flaw in a medium such as a conveyor belt, cable, rope,railroad track or road can be effected by sensing a Doppler shift in acarrier signal caused by a flaw.

This invention features a flaw detection system using acoustic Dopplereffect for detecting flaws in a medium wherein there is relative motionbetween the medium and system. There are transducer means, spaced fromthe medium to be inspected, for introducing to and sensing from themedium an acoustic signal that propagates in said medium at apredetermined frequency. There are also means, responsive to the sensedpropagating acoustic signal, for detecting in the sensed acoustic signalthe Doppler shifted frequency representative of a flaw in the medium.

In a preferred embodiment the transducer means may include a separatetransmitter and receiver. The transducer may be an ultrasonic transducerand the acoustic signal an ultrasonic signal. The transducer maytransmit an acoustic signal from propagation in the medium or thetransducer may transmit optical energy for inducing the acoustic signalin the medium. The transducer may include a laser for transmitting thatoptical energy. The transducer may include an acoustic receiver. Thetransducer may include an electromagnetic acoustic transmitter andreceiver for inducing an acoustic signal into the medium and sensing theDoppler shifted acoustic signal. The means for detecting may include aspectrum analyzer, or a bandpass filter and a peak detector, or an A toD converter and a digital filter for the purpose of distinguishing theDoppler effect frequency. In addition there may be a thresholdingcircuit identified with any one of the options for identifying apreselected label as a flaw. The medium to be inspected may be arailroad rail. The transducer may transmit to the surface of the mediumand receive from the surface of the medium an acoustic signal and theflaws detected may be surface flaws. Or the transmitter may induce anacoustic signal internally in the medium and the flaws detected may beinternal flaws. The transducer means may include a laser vibrometerinterferometer for sensing the acoustic signal propagating in themedium.

The invention also features a flaw detection system using acousticDoppler effect for detecting surface flaws when there is relative motionbetween the medium and system. There is an acoustic transducer meansspaced from the medium to be inspected for transmitting an acousticsignal to and receiving the reflected acoustic signal at a predeterminedfrequency from the surface of the medium to be inspected. Meansresponsive to the reflected acoustic signal distinguish the Dopplershifted frequency in the reflected acoustic signal from thepredetermined frequency of the transmitted acoustic signalrepresentative of a surface flaw in the medium.

The invention also features a flaw detection system using acousticDoppler effect for detecting flaws in a medium wherein there is relativemotion between the medium and system. There are transducer means spacedfrom the medium to be inspected for inducing an acoustic signal topropagate the medium at a predetermined frequency and sensing thepropagated acoustic signal in the medium. Means, responsive to thesensed propagating acoustic signal, distinguish the Doppler shiftedfrequency representative of a flaw in the medium.

In a preferred embodiment the transducer means may include anelectromagnetic acoustic transducer for inducing and sensing theacoustic signal. The transducer means may include a transmitter and areceiver and the transmitter may include a laser for locally heating themedium to generate acoustic signals.

DISCLOSURE OF PREFERRED EMBODIMENT

Other objects, features and advantages will occur to those skilled inthe art from the following description of a preferred embodiment and theaccompanying drawings, in which:

FIG. 1 is a schematic block diagram of a flaw detection system usingacoustic Doppler effect detection system according to this inventionadapted to inspect for defects in a railroad rail;

FIG. 2 is an enlarged detailed view of the acoustic transducer of FIG. 1implemented with separate receiver and transmitter in the ultrasonicrange;

FIG. 3 illustrates the output signal from the acoustic transducer;

FIG. 4A illustrates the output carrier signal reflected from theunflawed surface;

FIG. 4B illustrates the Doppler shifted return signal reflected from asurface flaw;

FIG. 4C is the magnitude spectrum of the signal reflected from theunflawed surface;

FIG. 4D is the magnitude spectrum of the reflection from the flaw;

FIG. 5 is a more detailed block diagram of one implementation of theDoppler detection circuit of FIG. 1;

FIG. 6 is a more detailed block diagram of another implementation of theDoppler detection circuit of FIG. 1;

FIG. 7 is a more detailed block diagram of another implementation of theDoppler detection circuit of FIG. 1;

FIG. 8 illustrates the peak magnitude of a Short Time Fourier Transformimplementation of the programmable digital signal processor of FIG. 7;

FIG. 9A illustrates the magnitude spectrum of the Doppler window;

FIG. 9B illustrates the inverse Fourier transform of the magnitudespectrum of FIG. 9A;

FIG. 9C is the result of convoluting the inverse Fourier transform ofFIG. 9B with the input signal of FIG. 3;

FIGS. 10A, B and C are schematic diagrams showing the acoustictransducer implemented with an electromagnetic acoustic transducer usingseparate spaced transmitter and receiver, separate adjacent transmitterand receiver, and a single combined transmitter/receiver unit,respectively; and

FIG. 11 is a schematic view of another form of transducer using a laseracoustic transmitter.

An advantageous feature of the high-speed flaw detection system usingacoustic Doppler effect according to this invention is that thetransducer need not, and in fact preferably is not, in contact with therail or other medium to be inspected. Instead, the transducer remotelysenses the discontinuities through the air. There are several devicesthat operate in a non-contact mode including electromagnetic acoustictransducers (EMAT) (G. A. Alers, Railroad Rail Flaw Detection SystemBased on Electromagnetic Acoustic Transducers, U.S. Department ofTransportation Report DOT/FRA/ORD-88-09 (1988) and laser-based acousticor ultrasound (LBU) (C. B. Scruby and L. E. Drain, Laser-Ultrasonics:Techniques and Applications, Adam Hilger, Briston, UK (1990)). Morerecently, air-coupled piezoelectric transducers have shown interestingresults in some materials (A. Safaeinili, O. I. Lobkis, nd D. E.Chimenti, “Air-coupled Ultrasonic Characterization of Composite Plates”,Materials Evaluation, Vol. 53, 1186-1190 (October 1995)). Air-coupledtransducers are attractive because they allow ultrasound to propagatethrough gaseous media without requiring mechanical contact between thetransducer and the medium to be inspected. When used for inspectingrailroad tracks the acoustic impedance mismatch between the steel andair is used to great advantage since it reflects most of the energy fromthe steel surface back to the transducer. When the invention is employedin railroad rail monitoring, a typical car speed for monitoring the railmay reach above sixty miles per hour and in fact, increased car speedleads to more pronounced Doppler effects and better overall efficiency.

There is shown in FIG. 1 a flaw detection system using acoustic Dopplereffect system according to this invention mounted on a railroad car 12,indicated generally in phantom, which moves in either direction asindicated by arrow 14 along railroad rail 16, which contains a flaw 18on its surface. System 10 includes a non-contact acoustic transducer 20which beams out an acoustic signal 22, referred to as a carrier signal,which reflects back from rail 16 as the returned or reflected signal 24.When signal 22 strikes the smooth portion of rail 16 the carrier signalcomes back with its frequency unchanged, but when acoustic signal 22strikes flaw 18 the return signal 24 will contain Doppler shiftedfrequency. Depending upon the position of the moving vehicle 12 and/orthe direction of the acoustic signal output 22, the Doppler shift may bean increase or a decrease in frequency. The return acoustic signal 24 issensed and transduced to an electric signal and submitted to Dopplerdetection circuit 26 which extracts the Doppler frequency and thensubmits it to a threshold or comparator circuit 28. If the Dopplerfrequency is above a preselected reference level a defect alarm outputis provided by threshold circuit 28.

When a transmitter and receiver are used in a bistatic arrangement,i.e., the transmitter and receiver are separated by a distance and ifthe angles between the transducers and the target (flaw) are ψ, theDoppler frequency shift can be expressed as: $\begin{matrix}{{\Delta\quad f} = {{\pm 2}{f_{s}\left( \frac{v_{s}}{c} \right)}\cos\quad\psi}} & (1)\end{matrix}$where f_(s) is the frequency of the input signal, Δf is the differencebetween the input frequency and the Doppler shifted frequency, v_(s) isthe relative speed between the system and the medium to be inspected, inthis case for example it may be the railroad car carrying the systemtraveling at for example sixty miles an hour, c is the wave speed in themedium, air in this case, and ψ is the angle between the direction ofmotion and the direction to the receiving transducer from the notch.

Acoustic transducer 20 may be a single transducer which acts as bothtransmitter and receiver, or it may be two separate units, one atransmitter, the other a receiver. In FIG. 2 transducer 20 a includessuch discrete devices where a transmitter 30 transmits an ultrasonicoutput beam 22 a and ultrasonic receiver 32 receives the reflectedultrasonic signal 24 a.

The output 40, FIG. 3, of transducer 20 shows a general smooth amplitudeprofile over time in the areas 42 but shows distinctive characteristics44 where a defect such as defect 18 has been seen. Typically the outputacoustic signal 22 b, FIG. 4A, is in the range of 100 kHz. Upon hittinga defect or flaw the return wave appears as at 24 b in FIG. 4B. Themagnitude spectrum 22 bb, FIG. 4C, of output signal 22 b shows a markedrise at 100 kHz while the magnitude spectrum 24 bb, FIG. 4D for thereturn signal 24 b is accompanied by a very distinct peak 50 at about115 kHz which is the Doppler shifted frequency resulting from theDoppler effect caused by the flaw 18.

Doppler detection circuit 26 may be implemented in any number of ways.For example, detection circuit 26 a, FIG. 5, may include an analogbandpass filter 60 which provides a bandpass window centered on 115 kHzwhere the Doppler shift is expected at a relative speed of 60 miles anhour between the medium and system. The output from filter 60 is thenselected by gated peak detector 62 so that any signals appearing in thatband above a certain level will be accepted as a flaw detection.Alternatively, Doppler detection circuit 26 b, FIG. 6, may include aspectrum analyzer 64 which directly provides the Doppler shiftedfrequency output.

In another implementation Doppler detection circuit 26 c, FIG. 7, mayinclude an analog to digital converter 66 which converts the analogsignal to a digital signal and then submits it to a programmable digitalsignal processor 68. The programmable digital signal processor may beprogrammed in a number of different ways. For example, it may beprogrammed to operate as a Short-Time Fourier transform. Beginning withthe signal as shown in FIG. 3, the Short-Time Fourier Transform$\begin{matrix}{{S_{\lambda}(f)} = {\frac{1}{2\pi}{\int_{- \infty}^{+ \infty}{{s(t)}{\mathbb{e}}^{{- {\mathbb{i}}}\quad{({2\pi\quad{ft}})}}{W_{\lambda}(t)}{\mathbb{d}t}}}}} & (2)\end{matrix}$results in discrete and prominent features 44 a, FIG. 8, correspondingto each of the flaws or defects 44 in FIG. 3.

Alternatively, the programmable digital signal processor 68 may beprogrammed to produce a bandpass 70, FIG. 9A, in the range of 105 to 115kHz then obtain the inverse transform 72, FIG. 9B, of response 70 andconvolve it with the return or reflected signal as shown in FIG. 3 inaccordance with the input signal designated x(n) and the filtercoefficient h(n) in the discrete-time domain directly as shown in thefollowing expression: $\begin{matrix}{{y(n)} = {\sum\limits_{k = 0}^{N - 1}{{h(k)} \times \left( {n - k} \right)}}} & (3)\end{matrix}$wherein y is the filtered output signal, N is the number of points, h isthe filter coefficient, k and n are index variables, and x is the inputsignal. The result of that convolution is shown in FIG. 9C wherein eachof the flaws or defects 44, FIG. 3, creates a discrete and veryprominent feature 44 b.

Although as disclosed herein the acoustic signals are continuous wavesignals, this is not a necessary limitation of the invention as pulse orspike pulse signals can also be used. For monitoring internal flaws 18b, c, d, FIGS. 10A, 10B and 10C, an electromagnetic acoustic transduceror EMAT may be used. Such a transducer 20 b, FIG. 10A, may include anelectromagnetic transmitter 30 b and receiver 32 b for monitoringinternal flaws such as flaw 18 b. While transmitter and receiver 30 band 32 b are spaced apart, FIG. 10A, this is not a necessary limitationfor as shown in FIG. 10B, EMAT transmitter 30 c and receiver 32 c may beadjacent to one another and it is not necessary that the receiver andtransmitter be separate, for as shown in FIG. 10C an EMAT transducer 20d which both transmits and receives can be used. As is well known, theEMAT transmitter 30 b establishes a varying magnetic field 31 b whichinduces the acoustic signal 22 b in rail 16. EMAT receiver 32 b throughits magnetic field 33 b senses the acoustic return signal 24 b.

Yet another transducer 20 e, FIG. 11, is shown in which the transmittermay be a laser 30 e that either provides its energy directly over beam22 e or through optical fiber cable 90 delivers its energy to rail 16where it induces an acoustic wave that propagates in rail 16. The lasershould be a powerful one such as a Q-switched Nd:YAG laser. The receiver32 e may be an EMAT transducer or an acoustic transducer as alreadydisclosed or may be an interferometer vibrometer device using aFabry-Perot technique, for example.

Although specific features of this invention are shown in some drawingsand not others, this is for convenience only as each feature may becombined with any or all of the other features in accordance with theinvention.

Other embodiments will occur to those skilled in the art and are withinthe following claims:

1. A flaw detection system using acoustic Doppler effect for detectingflaws in a medium wherein there is relative motion between the mediumand system comprising: air-coupled transducer means, spaced from themedium to be inspected, which transmit optical energy for introducing toand receiving from the medium an acoustic signal that propagates in saidmedium at a predetermined frequency; and means, responsive to thereceived propagating acoustic signal, for detecting in the receivedacoustic signal the Doppler shifted frequency representative of a flawin the medium.
 2. The flaw detection system using acoustic Dopplereffect of claim 1 in which said transducer means includes a laser fortransmitting said optical energy.
 3. A flaw detection system usingacoustic Doppler effect for detecting flaws in a medium wherein there isrelative motion between the medium and system comprising: air-coupledtransducer means, spaced from the medium to be inspected, forintroducing to and sensing from the medium an acoustic signal thatpropagates in said medium at a predetermined frequency said transducermeans including an acoustic receiver for sensing the acoustic signalpropagating in the medium and a transmitter that transmits opticalenergy.
 4. A flaw detection system using acoustic Doppler effect fordetecting flaws in a medium wherein there is relative motion between themedium and system comprising: air-coupled transducer means, spaced fromthe medium to be inspected, for inducing an acoustic signal to propagatein the medium at a predetermined frequency and receiving the propagatingacoustic signal in the medium; and said transducer means including atransmitter and a receiver and said transmitter including a laser forlocally heating the medium to generate acoustic signals; and means,responsive to the received propagating acoustic signal, fordistinguishing the Doppler shifted frequency representative of a flaw inthe medium.
 5. A flaw detection system using acoustic Doppler effect fordetecting flaws in a medium wherein there is relative motion between themedium and system comprising: an air-coupled transducer, spaced from themedium to be inspected, that transmits optical energy for introducing toand receiving from the medium an acoustic signal that propagates in saidmedium at a predetermined frequency; and a detector, responsive to thereceived propagating acoustic signal, that detects in the receivedacoustic signal the Doppler shifted frequency representative of a flawin the medium.
 6. The flaw detection system using acoustic Dopplereffect of claim 5 in which said transducer includes a laser thattransmits said optical energy.
 7. A flaw detection system using acousticDoppler effect for detecting flaws in a medium wherein there is relativemotion between the medium and system, comprising: an air-coupledtransducer, spaced from the medium to be inspected, that introduces toand senses from the medium an acoustic signal that propagates in saidmedium at a predetermined frequency, said transducer including anacoustic receiver that senses the acoustic signal propagating in themedium and a transmitter that transmits optical energy.
 8. A flawdetection system using acoustic Doppler effect for detecting flaws in amedium wherein there is relative motion between the medium and system,comprising: an air-coupled transducer, spaced from the medium to beinspected, that induces an acoustic signal to propagate in the medium ata predetermined frequency and receives the propagating acoustic signalin the medium, said transducer including a transmitter and a receiver,said transmitter including a laser that locally beats the medium togenerate acoustic signals; and means, responsive to the receivedpropagating acoustic signal, for distinguishing the Doppler shiftedfrequency representative of a flaw in the medium.